Some develop resistance. Others alter their behaviour. Mosquitoes of the genus Anopheles, the vectors of malaria, always find a way to foil human attempts to protect themselves from this disease. Researchers from the IRD and their partners have revealed their great capacity for adaptation, which weakens the strategies to combatting their presence, recommended by the WHO. A clinical trial conducted in some thirty or so villages in Benin demonstrated that, the combined use over an 18 month period of mosquito nets impregnated with deltamethrin, and another powerful insecticide in spray form inside the homes, did not lead to a decline in the disease. Neither the number of cases nor the prevalence of the infection among young children were reduced in comparison with the use of mosquito nets alone. In some localities, the introduction of nets led to a change in the feeding habits of the insects of the Anopheles genus, which usually bite at night time. They are now rife outside dwellings at dawn.
The long-term effectiveness of the current measures to prevent and combat the disease is therefore called into question. Scientists will once again need to innovate if we are to one day eliminate this disease for good.
On the recommendations of the WHO, 290 million impregnated mosquito nets were delivered in sub-Saharan Africa between 2008 and 2010, thus providing the means to protect 580 million people from malaria. At the same time 80 million people, or 10% of the endangered population, also had the walls of their homes sprayed with insecticides. Nevertheless, with 200 million people still affected every year and over 700,000 deaths all over the world, of which 80% in Africa, the disease continues to be a major threat to public health. The main obstacle to reducing the disease is the enormous ability of the mosquitoes that carry it to adapt to pyrethrinoids, the officially recommended insecticides. As revealed by recent studies undertaken by researchers from the IRD and its partners in Benin, the resistance of the main vector species, Anopheles gambiae, to these products is significantly increasing and weakening the strategies of the country's National Programme to Combat Malaria. The other major vector in Benin, Anopheles funestus, has for its part opted for another tactic: avoiding all contact with insecticides by modifying its feeding habits.
A combination of measures
A species with a particularly strong capacity for resistance, A. gambiae has succeeded in mutating genetically to withstand the increasing exposure to pyrethrinoids. To counter this phenomenon, the WHO has for several years been recommending the combined use of mosquito nets impregnated for long-term effectiveness, with carbamate sprays, another group of insecticides, inside the home. The latter act in a different way against the insects' nervous system, and are extremely powerful. Spraying these insecticides on the walls of dwellings constitutes a promising way of slowing down the appearance or development of resistance, due to the effect of the simultaneous exposure to different products.
The mosquito continues to mutate